Batteries are devices that convert chemical energy into electrical energy, as a result of chemical reactions between the negative electrode (cathode) and positive electrode (anode) via the intervening electrolyte. Electrode reactions may be described as an electrochemical couple that allows the passage of electrons or ionic species transfer from the anode to the cathode through the intervening electrolyte. The change from electronic conduction to ionic conduction occurs at the electrodes and involves electrochemical or Faradaic reactions. For the purposes of this application, the term conductor is intended to include the entire conducting system of a battery or any component thereof, but particularly the electrolyte and/or the electrodes.
The process of repeatedly or continuously converting chemical to electrical energy causes the battery to discharge, or deplete its capacity for producing electrical energy. Batteries can be classified as either primary or secondary. Primary batteries are assembled with high energy compounds such that the stored chemical energy can be withdrawn as electrical energy at some later time; primary batteries can be discarded without recharge. Secondary batteries are batteries developed to have a recharging capability. Materials used in secondary or storage batteries must be capable of maintaining electrode or electrolyte integrity over repeated charging and discharging cycles.
There are different classes of primary and secondary batteries, and a wide range of choices as to compatible pairs of anode and cathode processes for both primary and secondary batteries. Electrolytes may include, for example, aqueous salt solutions, non-aqueous electrolytes comprising either molten salts, organic or inorganic solvents rendered conductive by dissolved salts, or solid electrolytes. Again, by way of example, for primary batteries such as zinc manganese-dioxide, zinc-zinc chloride-manganese dioxide, zinc alkaline-manganese dioxide, zinc mercuric-oxide, zinc silver-oxide, or magnesium-manganese dioxide, the electrolyte solution is an aqueous solution of ammonium chloride (or zinc chloride), potassium hydroxide, sodium hydroxide, or magnesium perchlorate. For secondary batteries such as nickel cadmium, nickel-iron, nickel-zinc, silver-zinc, silver-cadmium, silver-iron, mercuric oxide-cadmium, or lead-acid, the electrolyte solution can be either potassium hydroxide or sulfuric acid.
Battery depletion may occur through use. More specifically, in today's conventional internal combustion vehicles, battery depletion typically occurs from electrical systems which include loads from clocks, radios, vehicle theft deterrent systems, engine system controls, and others. These loads draw current from a battery when it is not in use, and if sustained over a long period of time, these electrical components may deplete the battery's reserve capacity and render it completely discharged and unusable.
There is an increasing demand for more efficient primary and secondary batteries that will withstand heavy loads. Correspondingly, there is a need for a battery monitoring method that is capable of determining the state-of-charge instantaneously. The state-of-charge of a battery is the amount of available charge (amp hours) the battery can still usefully provide. Accurate monitoring of a battery prevents complete discharge, undercharging and overcharging of the battery, all of which are conditions that damage and/or limit the life of the battery.
There is a further need for a state-of-charge battery monitor to provide an accurate and instantaneous reading regarding the capacity of the battery to the user/operator of the system containing the battery. The user/operator must know the state-of-charge of a battery at all times because, as one example, if the battery's charge is depleted without sufficient warning, the operator of an electric vehicle may be stranded and unable to locate or reach a power source to recharge the battery. Therefore, there is a need for a metering system that reliably provides operators with an accurate and instantaneous indication of the measure of charge left in the battery, and consequently, an indication of the time remaining before recharging the battery is necessary.
More specifically, the current need for electric-powered systems or electric hybrid vehicles have renewed development interest in producing a lead-acid battery (a type of secondary, rechargeable battery) that is both efficient and easy to maintain. Lead-acid batteries, due to widespread availability and use in automotive, industrial, and consumer applications, are particularly desirable for electric-powered systems. The lead-acid battery has an efficient electrochemical system that is highly reversible and can be discharged and charged repeatedly before failure. Therefore, there is a particular need for a metering system that allows for continuous state-of-charge monitoring and that is electrically noninterfering, in the sense that the metering system does not draw or produce current from the battery and will not disturb other circuits in the electric or hybrid vehicle.
The problem of measuring the state-of-charge of primary and secondary batteries is not new. Known primary and secondary battery monitors have been directed to: (1) detecting the specific gravity of the battery electrolyte, (2) measuring the terminal voltages of the battery, and (3) tracking and monitoring the charge drawn from and supplied to the battery.
However, known systems for measuring the state-of-charge of a battery have been unsatisfactory in many areas. At best, these existing methods only yield approximate values for the state-of-charge because they have been designed to monitor and determine the total time of use, or rate of charge transfer of remaining battery capacity, without taking into account one or more of the following factors: ripple current, battery temperature, specific gravity of the electrolyte, and voltage potential across the battery cell terminals in an open circuit, etc. In addition, because these monitors typically monitor the state-of-charge from an electrical perspective, relying on the integration of the current in the battery, these monitors are often susceptible to interferences, and the readings obtained are not instantaneous. Thus, errors can accumulate, and the state-of-charge reading is not entirely accurate. In any case, no known battery monitor has employed optical technologies to determine the state-of-charge of a battery.
U.S. Pat. No. 5,321,389, issued Jun. 14, 1994 to Meister, discloses a battery charge monitor comprising an electronic circuit for connection to a vehicle battery. The circuit is responsive to a battery voltage to provide an electrical signal indicating a voltage drop across the terminals and uses relay coils to open and close the relay contacts for connecting and disconnecting the battery from the loads based on signals.
U.S. Pat. No. 5,321,626, issued Jun. 14, 1994 to Palladino, discloses an apparatus for monitoring and forecasting battery performance using a plurality of probe assemblies that sense output voltages and current across a series of connected batteries. The probe assemblies include a digital output device to display physical parameter information such as electrolyte specific gravity, temperature, individual battery voltage and electrolyte level, for comparison to corresponding fixed physical parameter values.
U.S. Pat. No. 5,339,017, issued Aug., 16, 1994 to Yang, discloses an arrangement of diodes and thyristors for detecting the state-of-charge of a vehicle battery. The individual diodes are arranged to define a predetermined forward voltage drop, which energizes the respective thyristors which energize respective light-emitting diodes (LEDs) to form a bar graph display of the state-of-charge of the battery. The greater the voltage drop across the various electrical loads, the greater the number of diodes which conduct and the greater the number of LEDs which are illuminated.
U.S. Pat. No. 5,345,163, issued Sep. 6, 1994 to Gibbons et al., describes a method for continuously monitoring battery charge level in three monitoring stages: in the first stage, overcharging is prevented if the monitored voltage exceeds a first predetermined voltage, as indicated by a "DO NOT CHARGE" signal; in the second stage, the "OK TO CHARGE" signal appears if the monitored voltage falls below a first predetermined voltage longer than a first predetermined period; in the third stage, the "OK TO CHARGE" signal is maintained until the monitored voltage falls below a second predetermined voltage for a period longer than a second predetermined period, resulting in the "MUST CHARGE" signal.
U.S. Pat. No. 5,381,096, issued Jan. 10, 1995 to Hirzel, discloses a metering system that monitors and communicates the state-of-charge of a battery-powered device to its operator. The apparatus measures the actual terminal voltage of the battery, and a digital electronic model computes the battery's charge by tracking an internal voltage which corresponds to the open circuit voltage of the battery. This internal voltage represents the battery's state-of-charge, and remaining battery life is shown on a visual display.
U.S. Pat. No. 5,483,165, issued Jan. 9, 1996, to Cameron et al. discloses a sense cell for determining remaining capacity and depletion condition of the main battery, based on the concept that by using a battery analogous to the main battery pack as the sense cell, the cell (being exposed to the same environmental conditions) will draw a larger load than that experienced by the main battery pack. Because the sense cell is identical to the cells in the main battery, the degradation of the sense cell and main battery cells is anticipated to be identical. Once the sense cell is fully depleted, a circuit prevents further current from being drawn from the main cells, and the operator is informed that recharging is necessary.
U.S. Pat. No. 5,656,919, issued Aug. 12, 1997 to Proctor et al., discloses a state-of-charge monitoring system for a battery under varying load and temperature conditions as the battery is being charged or discharged. Memory devices store discharge and charge values, and a processor determines the battery's state-of-charge. The Peukert equation is used to determine the battery's state-of-charge based on the battery's depletion condition.
U.S. Pat. No. 5,672,973, issued Sep. 30, 1997 to Arai et al., discloses an apparatus for monitoring the residual capacity of a battery by sensing the discharge current and terminal voltage based on a linear relationship between the terminal voltage and the current during discharge. The residual battery capacity is calculated by averaging the changes in the discharge current and the terminal voltage on a first time scale and obtaining linear functions based on the respective averages.
Other battery state-of-charge tracking devices and monitors are disclosed in the following references: U.S. Pat. No. 5,496,658 to Hein; U.S. Pat. No. 5,619,417 to Kendall; U.S. Pat. No. 5,281,955 to Reich; U.S. Pat. No. 5,047,961 to Simonsen; U.S. Pat. No. 4,949,046 to Seyfang; U.S. Pat. No. 4,947,123 to Minezawa; U.S. Pat. No. 5,633,592 to Lang; U.S. Pat. No. 4,931,738 to Maclntyre; U.S. Pat. No. 5,614,804 to Kayano; U.S. Pat No. 5,596,260 to Moravec; U.S. Pat. No. 5,352,982 to Nakazawa. Again, none of these use optical methods to determine the state-of-charge of a battery.
It is known to detect state-of-charge optically by measuring the changes in the refractive index of a solution (G. P. Hanckc, "A Fibre-Optic Sensor for Monitoring the State-of-Charge of a Lead Acid Battery," Proceedings of the IEEE Instrumentation and Measurement Technology Conference, Washington D.C., Apr. 25-27, 1989, pp. 496-489). Hancke describes a fiber-optic technique for determining the specific gravity of a lead-acid battery by monitoring leakage of light through the walls of a fiber contacting the electrolyte, converting the leakage to a related specific gravity value, and thereby determining the state-of-charge of a lead-acid battery. However, Hancke does not describe the use of optical absorption to measure battery charge.
Additionally, currently pending U.S. patent application, Ser. No. 08/566,340, discloses a fiber-optic refractive index monitor that detects small changes in the index of refraction of a liquid with an optical waveguide. The waveguide measures the change in light through a liquid-clad optical fiber having a liquid core, as an indication of state-of-charge of a lead-acid battery. Again, optical absorption is not used to measure battery charge.
Accordingly, there remains an unsolved need for a monitor capable of determining the actual state-of-charge of a battery and communicating that information to the user of the battery-powered system that is functionally useful with a wide range of battery types.